The demand for communications systems, such as networking and/or wireless solutions, is constantly increasing. As a result, the scope of networks (LAN or WAN) can be increased to include a variety of devices, such as a camera system, a wireless interface, an RFID reader, and/or a variety of other devices. The devices that connect to a network may require power in order to operate. Such power can come from, for example, batteries or from being plugged into a standard wall outlet to receive AC power. However, batteries have only a limited operating life before they must be recharged or replaced. In addition, power that is provided from an AC power outlet is typically converted to DC power by either an internal or external power supply. Such power conversion can include a substantial amount of additional circuitry that introduces additional cost, complexity, and occupied space.
One solution to providing power to components on an Ethernet network is power-over-Ethernet (PoE), which is governed in part by IEEE Standard 802.3 and other relevant standards. In PoE, both power and data are distributed over Ethernet cables between devices on the network. For example, a given device can include power sourcing equipment (PSE) that provides a current that is carried across an Ethernet cable to a powered device (PD) on the Ethernet network. The distribution of power over Ethernet cables, such that both power and data are distributed to devices on the network, allows for reduced costs of installation and reduced need for power conversion components and power cables, as well as a variety of other advantages.
FIG. 1 illustrates an example of a typical network 10 that provides power via PoE. The network 10 includes a first communication device 12 and a second communication device 14. For example, the first communication device 12 could be a data server and the second communication device 14 could be a wireless router. The first communication device 12 includes power sourcing equipment (PSE) 16 that is configured to provide power. For example, the PSE 16 can be a 48 volt DC power supply that can provide up to 350 milliamps (mA) of current. The PSE 16 can also include switching components that are configured to detect an impedance provided by an attached load, such that the provided power is only supplied upon detecting the impedance of the attached load.
The network 10 also includes a first data transformer 18, a second data transformer 20, a third data transformer 22, and a fourth data transformer 24 coupled to a first connector 26 of an Ethernet cable 28. The data transformers 18, 20, 22, and 24 provide signal isolation between a data bus 30 and the Ethernet cable 28. In the example of FIG. 1, the PSE 16 has a positive terminal that is coupled to a center tap of the secondary of the first data transformer 18 and a negative terminal that is coupled to a center tap of the secondary of the second data transformer 20. As described in greater detail below, it is to be understood that the PSE 16 is not limited to being coupled to the first and second data transformers 18 and 20, but both the positive terminal and the negative terminal of the PSE 16 could instead be coupled to any two of the data transformers 18, 20, 22, and 24.
The data provided to and from the data bus 30 travels across the Ethernet cable 28 via four sets of wire pairs 32 between the first connector 26 and a second connector 34 that is coupled to the second communication device 14. In the example of FIG. 1, the wire pairs 32 are depicted as twisted data lines, such as can be typical in a given Ethernet cable. In addition, as demonstrated in the example of FIG. 1, the power provided by the PSE 16 also travels across the Ethernet cable 28 via two sets of the wire pairs 32, with a current supply path between the terminals 1 and 2 of the connectors 26 and 34 and a current return path between terminals 3 and 6 of the connectors 26 and 34. The second communication device 14 includes a first data transformer 36, a second data transformer 38, a third data transformer 40, and a fourth data transformer 42. The data transformers 36, 38, 40, and 42 provide signal isolation between a data bus 44 and the Ethernet cable 28, such that data is transferred between the first communication device 12 and the second communication device 14 via the data buses 30 and 44. It is to be understood that the network 10 in the example of FIG. 1 is not intended to be limited to having all four of the data transformers 18, 20, 22, and 24, and all four of the data transformers 36, 38, 40, and 42. For example, the first communication device 12 and the second communication device 14 could instead include only two data transformers each, such that the data provided between the data buses 30 and 44 could propagate on only two of the four wire pairs 32, rendering the other two wire pairs 32 as spares. The PSE 16 could thus provide and receive current on the two wire pairs 32 that propagate the data, the spare wire pairs 32, or a combination thereof.
The second communication device 14 includes a powered device (PD) 46. The PD 46 is a load for the current provided by the PSE 16, such that the PD 46 can provide operating power for the second communication device 14. As an example, the PD 46 can include one or more components that provide an indication to the PSE 16 that it is coupled and ready to consume power provided by the PSE 16, such that the PSE 16 can be switched-on in response, as described above. In addition, the PD 46 can include or can be coupled to a DC/DC converter (not shown) that can convert the 48 VDC power provided by the PSE 16 to a lesser voltage for use by the second communication device 14. A capacitor 47 decouples the input power terminals of the PD 46.
The second communication device 14 also includes a first diode bridge 48 and a second diode bridge 50. The first diode bridge 48 can be fabricated on a first semiconductor die 52 and the second diode bridge 50 can be fabricated on a second semiconductor die 54. The first diode bridge 48 is coupled to the center taps of the first and second data transformers 36 and 38, as well as a positive input power terminal of the PD 46 at a node 56 and a negative input power terminal of the PD 46 at a node 58. As such, the supply current from the PSE 16 flows through the first diode bridge 48 to the PD 46, and the return current from the PD 46 flows through the first diode bridge 48 to the PSE 16. However, it is to be understood that the first diode bridge 48 is configured such that it is polarity insensitive. Specifically, the PD 46 can receive the supply current even if the polarity of the PSE 16 is switched, such that the supply current and the return current each flow through different wire pairs 32 without an effect on the PD 46 receiving the supply current. In a similar manner, the second diode bridge 50 is coupled to the center taps of the third and fourth data transformers 40 and 42, as well as a positive input power terminal of the PD 46 at the node 56 and a negative input power terminal of the PD 46 at the node 58. The second diode bridge 50 is likewise polarity insensitive. As a result, the positive terminal and the negative terminal of the PSE 16 can each be coupled to any two of the data transformers 18, 20, 22, and 24 and still provide the supply current to the positive input power terminal of the PD 46.
Due to constraints imposed on the wire pairs 32 as set by relevant PoE standards, each conductor of a given one of the wire pairs 32 is allowed to conduct a maximum amount of current. For example, as dictated by the 802.3af PoE standard, a given one of the wire pairs 32 is allowed to conduct up to approximately 175 mA of current, such that the PSE 16 is capable of supplying a maximum current of approximately 350 mA. An additional current of approximately 10.5 mA may also be present on a given conductor for current balancing between the two conductors of a given wire pair. However, as networks become more complex and PoE applications become more prevalent, communication devices that are configured to receive power via PoE may require additional current. As described above, a given set of wire pairs 32 may be limited in the amount of current they can provide, and thus cannot be configured to provide more current.
One solution could be to add an additional PSE, such as at the third and fourth data transformers 22 and 24 in the example of FIG. 1. However, a single load, such as the PD 46 in the example of FIG. 1, would draw current non-uniformly through each of the diode bridges 48 and 50 due to variations caused by a negative temperature coefficient inherent to the operation of diodes. As a result, substantially all of the current would be drawn from one of the two diode bridges 48 and 50 by the PD 46, causing overheating of wires, data transformers, and/or other components, as well as causing the current output of the PSE 16 to trip off. Accordingly, an additional PD 46 may be required to provide an increased amount of power to the second communication device 14. However, the addition of another PD 46 necessitates an additional DC/DC converter along with current sharing circuitry, which results in an increase in cost and circuit complexity of the second communication device 14.